<<snip>>

> This makes the design very complex as one needs a
> high speed VCO that controls the clock of the SCF and two analog tracking
> filters that have to follow the high speed VCO frequency.

<<snip>>

> From my point of view it's too costly for
> what you get.

I'll echo Dieter's sentiments on this - yes it can be done, but you have to work very hard to make it happen, so in the end it is much easier to make a voltage-controlled filter using more conventional means, i.e. transconductance amps etc. My efforts with the MF10/LMF100 may give an idea of what's involved (though I did try and push the clocking frequency beyond the normal audio range):

http://www.timstinchcombe.co.uk/synth/mf10/mf10.html

Tim